1. Field of the Invention
The present invention generally relates to a single focusing lens and more particularly to a single focusing lens for an optical disc recording and/or reproducing apparatus.
2. Description of the Prior Art
It has previously been known that a single focusing lens for a high density recording optical disc recording and/or reproducing apparatus is required to be a lens wherein both spherical aberration and coma aberration are satisfactorily corrected, that is, an aplanatic lens.
In case of constituting this focusing lens by a single lens (glass molded lens), both surfaces thereof are made to be aspherical surfaces. But, in this case, there arises coma aberration due to a shift of the optical axis (decentration) between the two aspherical surfaces so that the focusing lens must be designed so as to minimize decentering aberration.
However, such a single focusing lens becomes prone to decentering aberration as its numerical aperture becomes a large value of about 0.6, so that it has been impossible to practically provide the single focusing lens having a large numerical aperture and a small decentering aberration.